STABILIZATION OF CYCLIC ACETALS

Abstract: A method for the stabilization of cyclic acetals against oxidation, reduction and decomposition for extended periods of time by the addition thereto of a tertiary amine, wherein the cyclic acetals may be polymerized without prior removal of the tertiary amine.

Description: STABILIZATION OP CYCLIC ACETALS Field of the Invention The present invention relates to the stabilization of cyclic acetals utilizing trace amounts of an organic base. More particularly, the invention relates to the stabilization of cyclic acetals using tertiary amines. Backgro nd of the Invention Cyclic acetals may be utilized as a raw material for the production of oxymethylene polymers as well as other industrial materials. For example, in the production of oxymethylene polymers from trioxane, a cyclic acetal, it is critical that the trioxane be of high purity. It is well known that trioxane is unstable in the presence of oxygen, particularly when it is in the molten state. The presence of even a slight amount of oxygen will oxidize and decompose molten trioxane to form formic acid and formaldehyde. Generally, trioxane may be stored for periods of six months or more as a solid material prior to its use for the production of oxymethylene polymers. During this period, polyoxymethylene chains tend to form in the stored trioxane. These polyoxymethylene chains are of significant molecular weight and tend to be insoluble in trioxane. The chains do not readily melt at trioxane processing temperatures which may cause plugging of processing equipment and significant loss of trioxane-starting material. For commercial usage, it would be advantageous to store trioxane for periods of six months or more. Current methods do not satisfactorily stabilize trioxane against oxidation and decomposition during transportation and storage for long periods of time. Oxidation and decomposition may be prevented by preserving trioxane using bases, metal salts and antioxidants. However, problems arise when attempting to store the preserved trioxane so that it can be utilized for the production of oxymethylene polymers without removal of the preservative. To prevent formation of impurities during storage, base stabilizers have been added to trioxane. U.S. Patent No. 4,125,540 to Sugio et al. suggests a process for the stabilization of trioxane in which a trivalent organic phosphorus compound is added to the trioxane in an amount of 10 to 1,000 ppm, based on the weight of trioxane. U.S. Patent No. 4,081,591 to Takiyama et al. suggests a method for the stabilization of unsaturated cycloacetal resins against the formation of unsaturated aldehydes by the addition of a base such as a secondary or tertiary amine in the amount of 5 parts by weight per 100 parts by weight of the unsaturated cycloacetal resin. U.S. Patent No. 4,453,752 to Naito et al. suggests the addition of a base, such as triethylamine, at the end point of trioxane polymerization reaction to stop the reaction by neutralization of the catalyst. The '752 reference is primarily concerned with separation and recovery of unreacted trioxane fromthe solvent-amine-unreacted trioxane system resulting from neutralization of the polymerization catalyst, and employs a specific distillation-separation process to achieve the separation. Attempts have been made to stabilize trioxane using relatively high concentrations of strong organic bases. However, the presence of such bases may adversely affect the polymerization reactivity of trioxane, particularly since the base may reacts to neutralize the polymerization catalyst. It is also known to stabilize trioxane against the formation of formic acid with phenolic antioxidants or disulfides. However, these additives do not prevent the formation of polyoxymethylene upon ..solidification of trioxane. Therefore, it becomes necessary to remove the base from trioxane before polymerization which results in undesirable, additional process steps. It is generally known that trioxane may be stabilized against conversion to polyoxymethylene by the addition of water. However, water is only a semieffective stabilizer and because of adverse effects must be removed prior to the polymerization of trioxane. It has now been discovered that cyclic acetals may be stabilized so as to minimize the formation of insoluble materials during transportation and storage for extended periods of time, while at the same time preserving the cyclic acetals in a form such that it then may be reacted to produce quality polymers without prior separation of the stabilizer. Summary nf the Invention The present invention is directed to a method for the stabilization of cyclic acetals against oxidation, reduction and decomposition for extended periods of time, characterized by admixing a stabilizing amount of at least one tertiary amine or derivative thereof with cyclic acetal, wherein the cyclic acetal may be utilized as starting materials in polymerization processes without purification to remove the tertiary amine. The invention is further directed to cyclic acetals stabilized against oxidation, reduction and decomposition characterized as an admixture of cyclic acetals and at least about 0.1 parts per million of a tertiary amine, based on the total parts of cyclic acetals, wherein the cyclic acetals may be utilized as starting materials in polymerization processes without purification to remove the tertiary amine. Detailed Descripr.ion of the Invention Cyclic acetals are compounds having a ring system wherein there is contained at least one radical of the formula: Generally, cyclic acetals containing the aforementioned radical are suitable for stabilization according to the method of the present invention. Particularly, preferred cyclic acetals which may be stabilized according to the method of the invention include saturated acetals selected from 1, 3, 5-trioxane, 1, 3, 5,7-tetroxane, 1,3-dioxolane, 1, 3, 5-trioxepane, 1,3-dioxepane and mixtures thereof. The present invention relates to a stabilization method characterized by the addition of a small amount of an organic base, typically a tertiary amine, to cyclic acetals which do not react with formaldehyde. When admixed with cyclic acetals, the tertiary amine prevents oxidation, reduction and decomposition for long periods of time. Thereafter, the cyclic acetals may be utilized in industrial processes, e.g., polymerization of 1,3,5-trioxane to oxymethylene polymers, without prior separation of the amine therefrom. Typically, tertiary amines useful as stabilizers in the invention include triethanolamine, N-methylmorpholine, ethyldiethylamine, ributylethanolamine, trioctylethylamine and mixtures thereof. Additional suitable organic, tertiary amines are dimethylaminoet hanol , diethylaminoethanol , phenyldiethylamine, ethanolphenylethylamine , tri-isopropanolamine, tripropanolamine, dimethylcyclohexylamine, diethylcyclohexylamine, pyridine, tributylamine and mixtures thereof. Generally, the tertiary amines may be admixed with cyclic acetals in a concentration of at least about 0.1 part per million, based on the total parts of cyclic acetals. Typically, the tertiary amines may be admixed with cyclic acetals in concentrations of from about 0.1 to about 10 parts per million, based on the total parts of cyclic acetals, and preferably in concentrations of from about 0.5 to about 5 parts per million (e.g., tertiary amines concentration of about 2 ppm) . Tertiary amine concentrations greater than about 10 ppm may result in incomplete reaction of cyclic acetals during polymerization due to neutralization of the catalyst. The tertiary amines of the present invention are of an intermediate strength as indicated by the dissociation constant, pκ a , at 25°C in H 2 0. Generally, a pK a value from about 7.0 to about 9.5 is necessary to achieve the benefits of stabilization. Typically, the pK « value is from about 7.5 to about 9.0, and preferably, pK. value is about 7.9. In the polymerization of trioxane, wherein stabilized trioxane in the molten state (the melting point of trioxane is about 62°C) is introduced into a polymerization reactor, it is important that the tertiary amine exhibits a boiling temperature above about 62°C, and preferably above about 70°C, to ensure that the tertiary amine remains in the trioxane after it is melted and fed to the polymerization reactor. It has also been discovered that stabilization may be further improved by the addition of a small amount of an antioxidant to stabilized cyclic acetals of the present invention. Antioxidants that may be added to the stabilized cyclic acetals of the invention are generally known by those skilled in the art. Useful antioxidants may be selected from 2, β-di-tert-butyl-4-methylphenol, 2,2'-methylene-bis (4- ethyl-6-tert-butylphenol) , 1, 6-hexamethylene-bis- (3,5-dι- tert-butyl-4-hydroxyhydrocinnamate) , triethyleneglycol-bis- 3 (3-tert-butyl-4-hydroxy-5-methylphenol) propionate, 1,3,5- tris (4-tert-butyl-3-hydroxy-2 , 6-dimethylbenzyl) -1, 3 , 5- triazine-2,4, 6 (1H,3H,5H) trione, 1,1,3-tris (2-methyl-4- hydroxy-5-tert-butylphenyl) butane, tetrakis[methylene (3,5- di-tert-butyl-4-hydroxycinnamate) ] methane, and mixtures thereof. These antioxidants are generally added to stabilized cyclic acetals in an amount from about 0.1 to about 1.0 weight percent, based on the total weight of the acetals, preferable from about 0.2 to about 0.5 weight percent. The following examples are general illustrations of the method of preparing stabilized cyclic acetals in accordance with the present invention; additional cyclic acetals within the scope of the invention containing an organic base, e.g. , tertiary amine, and an antioxidant may be prepared using similar procedures. Example 1 Trioxane utilized herein was prepared by cyclic trimerization of formaldehyde. This trioxane was admixed with sodium, a strong metallic base, and benzophenone under a nitrogen blanket to remove any protonic impurities, e.g., water. The strong metallic base also catalyzed a formose reaction that tied up any monomeric formaldehyde. Afterwards, the trioxane was distilled to separate it from the sodium. The distilled, molten trioxane was transferred under a nitrogen blanket to heated 7 ml glass vials sealed with Teflon® lined caps via a 10 ml heated glass syringe and 8 an 18 gauge stainless steel needle. Typical analysis of the trioxane after transfer showed less than 30 ppm water content and less than 20 ppm formaldehyde content. Two amines, triethanolamine and N-methylmorpholine, were diluted to 0.5 weight percent in trioxane, and appropriately sized aliquants of the dilute trioxane-amine solutions were added to samples of the unstabilized, molten trioxane to yield final compositions. These samples were compared against a blank, which consisted of trioxane containing no amine. During these experiments, formaldehyde concentration in the stabilized trioxane samples was monitored to determine the amount of oxidation, reduction and decomposition. Increasing formaldehyde levels in the stored samples, identified by the amount of solids formation, indicated that the cyclic acetals were decomposing, and that solids (i.e., impurities) were forming as the trioxane went through a phase change. A visual, comparative scale was established to determine the amount of solids formation in the samples. The scale was based upon ratings of 1, indicating no visible solids formation, through 6, indicating complete solids formation. A visual level of 3 indicated about 50% solids formation. The samples were stored in the solid phase and remelted (at about 62°C) to determine solids. An additive was rejected as a stabilizer if a solids level greater than 3 was indicated during the testing. Table 1, below, illustrates the effects of the additives on long term storage of stabilized trioxane. TABLE 1 EFFECTS OF ADDITIVES ON LONG TERM STORAGE OF TRIOXANE Trioxane samples containing 3 and 6 ppm of cha NW-IP additive showed low solids levels afcer 32 weeks of storage ac ambient temperatures while the sanφle concaining no addieive. i.e.. chβ blank, showed a solids level of over about 50 weighc percenc. Exaπrnlt ? Trioxane samples prepared according co chβ procedures of Example 1. abova, were heac scrassed ac 100 β c for concroll β d periods of time. Heac scressing of cyclic ac β cal β , e.g.. crioxane, ae temperatures just below che boiling poinc cend co accalerace che formation of formaldehyde which resulcs in significancly less ci β to d β c β πnin β β cabilicy effeccs in accordance wich these examples. Trioxane samples stabilized wich NMMP and blanks, scored for 32 weeks ac ambiβnc cβmpβraeurβ. were heat scrassed ac a temperature of 100°C for periods of 45 eo 360 minuc β s prior co solids level and cloud poinc measurements. 'NMMP: N-mechylmorpholine The samples were stored as solids at ambient temperature and remelted prior to determining solids levels. The cloud point, reaction time that polymer precipitation occurs after addition of the catalyst, was determined after the final heat stress measurements; 0.0093 weight percent (i.e., 93 ppm of boron trifluoride dibutyletherate) of polymerization catalyst, was added to the molten sample. The time at which cloudiness appeared, i.e., initial polymerization, is the cloud point. All polymerizations were run in stirred tube reactors using 5 ml. of trioxane and 0.16 ml of 1,3- dioxolane. Screw-top septum capped vials were used as reactors and magnetic stirrer bars provided mixing. Reagents were directly injected through the septum in the screw cap. The catalyst solution was injected directly into the molten trioxane. The reactors were heated by hot oil baths maintained at 70°C. Table 2, below, indicates the results of this trial. 11 TABLE 2 EFFECTS OF ADDITIVES ON STORED, HEAT STRESSED TRIOXANE Example 3 To determine the ability of the stabilizers to prevent solids from forming upon recrystallization, unstabilized and stabilized trioxane samples underwent 3 heat stress cycles at 100°C for 120 minutes. Molten trioxane was stabilized by admixing it with N-methylmorpholine and triethynolamine, and heated to 100°C for 2 hours. The molten, stabilized trioxane was cooled to ambient temperature and remelted at 70°C to determine the solids contents. After each heating cycle, the samples were crystallized, remelted at 70°C and measured for solids. Cloud points were determined in accordance with the procedures described in Example 2, above. Table 3, below, presents the effects of additives on heat stressed trioxane and cloud points. Not measured 12 TABLE 3 EFFECTS OF ADDITIVES IN HEAT STRESSED TRIOXANE 3 TEOA: Triethanolamine

Claims: CLAIMS We Claim: 1. A method for the stabilization of cyclic acetals against oxidation, reduction and decomposition for extended periods of time, comprising adding to cyclic acetals a stabilizing amount of at least one tertiary amine or derivative thereof. 2. The method for the stabilization of cyclic acetals according to Claim 1, wherein the cyclic acetals are selected from the group consisting of 1, 3, 5- trioxane, 1, 3, 5, 7-tetroxane, 1, 3-dioxolane, 1, 3, 5-trioxepane, 1, 3-dioxepane and mixtures thereof. 3. The method for the stabilization of cyclic acetals according to Claim 2, wherein the tertiary amine is selected from the group consisting of triethanolamine, N-methylmorpholine, ethyldiethylamine, tributylethanolamine, trioctylethylamine, dimethylaminoe hanol , diethylaminoethanol , phenyldiethylamine, ethanolphenylethylamine, tri- isopropanolamine , t ripropanolamine , dimethylcyclohexylamine, diethylcyclohexylamine, pyridine, tributylamine, and mixtures thereof. 4. The method for the stabilization of cyclic acetals according to Claim 3 , wherein the amount of tertiary amine is at least about 0.1 parts per million, based on the total parts of cyclic acetals. 5. The method for the stabilization of cyclic acetals 14 according to Claim 4, wherein the amount of tertiary amine is from about 0.1 to about 10 parts per million. 6. The method for the stabilization of cyclic acetals according to Claim 5, wherein the amount of tertiary amine is from about 0.5 to about 5 parts per million, based on the total parts of cyclic acetals. 7. The method for the stabilization of cyclic acetals according to Claim 6, wherein the tertiary amine exhibits a pK. from about 7.5 to about 9. 8. The method for the stabilization of cyclic acetals according to Claim 7, further comprising the step of adding an antioxidant to the cyclic acetals. 9. The method for the stabilization of cyclic acetals according to Claim 8, wherein the antioxidant is selected from the group consisting of 2,6-di-tert- butyl-4-methylphenol, 2,2'-methylene-bis(4-ethyl-6- tert-butylphenol) , 1,6-hexamethylene-bis-(3,5-di-tert- butyl-4-hydroxyhydrocinnamate) , triethyleneglycol-bis- 3 (3-tert-butyl-4-hydroxy-5-methylphenol) propionate, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)- 1,3,5-triazine-2,4,6 (1H,3H,5H) trione, 1,1,3-tris (2- methyl-4-hydroxy-5-tert-butylphenyl) butane, tetrakis[methylene (3, 5-di-tert-butyl-4- hydroxycinnamate) ] methane, and mixtures thereof. 10. The method for the stabilization of cyclic acetals according to Claim 9, wherein the antioxidant is added 15 in the amount from about 0.1 to about 1.0 weight percent, based on the total weight of the cyclic acetals. 11. A method for the stabilization of saturated, cyclic acetals against oxidation, reduction and decomposition for extended periods of time, comprising admixing a saturated, cyclic acetal selected from the group consisting of 1,3,5-trioxane, 1,3,5,7-tetroxane, 1,3- dioxolane, 1,3,5-trioκepane, 1,3-dioxepane and mixtures thereof, and from about 0.1 to about 10 parts per million, based on the total parts of cyclic acetal, of an organic, tertiary amine selected from the group consisting of triethanolamine, N- methylmorpholine, ethyldiethylamine, tributylethanolamine, trioctylethylamine , dimethylaminoethanol , diethylaminoethanol , phenyldiethylamine, ethanolphenylethylamine, tri- isopropanolamine, tripropanolamine, dimethylcyclohexylamine, diethylcyclohexylamine, pyridine, tributylamine, and mixtures thereof, wherein the cyclic acetals may be utilized as starting materials in polymerization processes without purification to remove the tertiary amine. 12. Cyclic acetals stabilized against oxidation, reduction and decomposition for extended periods of time, comprising an admixture of cyclic acetals and at least about 0.1 parts per million of a tertiary amine, based on the total parts of cyclic acetals, wherein the cyclic acetals are utilized as starting materials in 16 polymerization processes without purification to remove the tertiary amine. 13. The cyclic acetals according to Claim 12, wherein the cyclic acetals are selected from the group consisting of 1, 3, 5-trioxane, 1, 3 , 5 , 7-tetroxane, 1, 3-dioxolane, 1, 3 , 5-trioxepane, 1, 3-dioxepane and mixtures thereof. 14. The cyclic acetals according to Claim 13, wherein the tertiary amine is selected from the group consisting of t r iethanolamine , N-methylmorphol ine , ethyldiethy lamine , t r ibuty 1 et hano 1 amine , t r iocty lethy lamine , dimet hy laminoet hano 1 , diethylaminoethanol, phenyldi ethyl amine, e t hano lpheny lethy lamine, tri-isopropanolamine, tripropanolamine, dime thylcyclohexy lamine , diethylcyclohexylamine, pyridine, tributy lamine, and mixtures thereof. 15. The cyclic acetals according to Claim 14, wherein the amount of tertiary amine is from about 0.1 to about 10 parts per million, based on the total parts of cyclic acetals. 16. The cyclic acetals according to Claim 15, wherein the amount of tertiary amine is from about 0.5 to about 5 parts per million, based on the total parts of cyclic acetals. 17. The cyclic acetals according to Claim 16, wherein the tertiary amine exhibits a pK. from about 7.5 to about 17 18. The cyclic acetals according to Claim 17 wherein the tertiary amine exhibits a pK a of about 7.9. 19. The cyclic acetals according to Claim 18, further containing an antioxidant in an amount from about 0.1 to about 1.0 weight percent, based on the total weight of the acetals. 20. The cyclic acetals according to Claim 18, wherein the antioxidant is selected from 2, 6-di-tert-butyl-4- methylphenol, 2,2 '-methylene-bis (4-ethyl-6-tert- butylphenol) , 1, 6-hexamethylene-bis- (3,5-di-tert- butyl-4-hydroxyhydrocinnamate) , triethyleneglycol-bis- 3 (3-tert-butyl-4-hydroxy-5-methylphenol) propionate, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)- l,3,5-triazine-2,4,6 (1H,3H,5H) trione, 1,1,3-tris (2- methyl-4-hydroxy-5-tert-butylphenyl) butane, tetrakis tmethylene (3 , 5-di-t-butyl-4- hydroxycinna ate) ] methane, and mixtures thereof.

Inventors: STAUTZENBERGER ADIN L, YEARWOOD MICHAEL G, YOUNG DARYL A, STAUTZENBERGER, ADIN, L., YEARWOOD, MICHAEL, G., YOUNG, DARYL, A.
Applicants: HOECHST CELANESE CORP, HOECHST CELANESE CORPORATION
Application year: 1994
Main classification: C07D317
IPC codes: C07D317, C07D323